Brick stoves and fireplaces with multiple or tortuous flues have been used in Europe and Asia for many years, one such structure being commonly referred to as a Russian Stove. Russian Stoves are massive brick and mortar structures which have a series of vertically and/or horizontally disposed flues alternating back and forth to provide a long flow path for products of combustion. In a Swedish stove design, the firebox has vertically extending walls for directing the products of combustion upward into a flue chamber having passageways for carrying these combustion gases downward past exterior portions of the firebox to outlet means located near the bottom of the flue chamber. A similar flue arrangement has been suggested for use in a hot air furnace as shown in U.S. Pat. No. 700,664 to Lee, et al. These flue arrangements are commonly referred to as "Fountain" designs.
The relatively long flow path in these designs allows the surrounding masonry to absorb approximately 80 to 90 percent of the combustion heat and store it for gradual release to a room or other space for a long period after the combustion process has ended. The heat source may be a rapidly burning wood fire which may reach sufficiently high temperatures (about 1,000.degree. F. or greater) to thoroughly burn up all combustible solids and liquids so that there is very little smoke and little or no creosote buildup in the flues. The wood pieces are usually small (less than about 21/2 to 3 inches in diameter) to insure a fast burn. Combustion products leave the stove and pass to ambient air by natural convection through an exhaust flue provided by a chimney or stovepipe connected to the stove flues. When the fire has died out at the end of the combustion cycle, which may take as little as twenty to thirty minutes, a damper in the chimney or stovepipe is shut so as to close off the stove flues and reduce the amount of hot air escaping through the exhaust flue. There need be no damper or other mechanism for controlling the entrance of fresh air into the firebox.
The heat generated by such fast burns will cause exterior surfaces of the surrounding masonry structure to reach peak temperature (about 150.degree. F. to 180.degree. F.) within about five hours after a rapid burn has been completed. While not hot enough to burn the skin, these temperatures are hot enough to preclude resting a hand on heated exterior surfaces for more than a few seconds. The large mass of these masonry structures, which may contain six or more tons of brick, will retain considerable heat and may provide steady radiant and convection heat for a room or other space for 12 hours or more. On particularly cold days, two or more consecutive refuelings one after another during a single firing period may be needed to bring the surrounding bricks to their peak temperature before closing the damper. In addition, a second firing period may be needed during a 24 hour period. The number of firings needed per day also depends on the level of insulation in the building being heated. For a well insulated building and normal winter temperatures, one or two rapid burns in a single firing period may be sufficient to provide enough heat for a full day (24 hours).
Prior art masonry fireplaces and stoves have the disadvantage of requiring large numbers of bricks which must be secured together by a bonding material, such as portland cement and the like, and which must be assembled by highly skilled craftsman in accordance with precise construction requirements.
These masonry structures may require two to four weeks for construction alone. In addition, conventional bonding materials, such as portland cement, may have to be cured for an additional three or four weeks after assembly of the stove before it can be fired for actual use as a source of heat.
A further disadvantage of prior art structures is that the masonry firebox is rigidly secured along two or more sides to the flue forming masonry by mortar or other brick bonding material such that thermal expansion and contraction of the firebox relative to the flue masonry can produce excessive thermal stress and unsafe and/or unsightly cracks and/or fractures in the masonry structure. Similarly, the masonry cap or other overlying structure in the Swedish or Fountain design is rigidly secured to the outer wall of the flue chamber. This can also produce excessive thermal stress and cracking, particularly since extremely hot combustion products may impinge directly on the overhead structure and cause this portion of the flue chamber enclosure to expand more rapidly than the remainder during fast-burn firings.
In a masonry stove, the heat energy produced by a hot, fast burning fire established once or twice a day is absorbed by the relatively dense mass of the masonry material which acts as a heat sink so that the walls of the mass then slowly release the stored heat to the surrounding air. Therefore, a principal advantage of a masonry stove is its ability to supply heat over a long period of time after a relatively brief firing. In marked contrast to masonry heating structures, metal stoves and furnaces extract only about 60 percent or less of the heat generated by fuel combustion. Because their heat storage capabilities are relatively limited due to lack of heat storage mass, metal heating systems must be frequently fired to establish a relatively constant level of heat in an adjacent or remote space. Furthermore, woodburning metal units require regulation of the combustion air for controlled burning of the wood so that heat can be supplied over a longer period without excessive fuel consumption. This reduces combustion efficiency and results in a buildup of creosote in the flues and other passageways of such units.